The present invention relates to a method for drying of material, especially of plastic granules, wherein said material disposed in a container is dried by drying air which is adjusted to a drying temperature specific to said material to be dried.
It has been known that thermoplastic materials are more or less hygroscopic. The absorbed moisture, usually stemming from the atmosphere, may cause technical and manufacturing problems during the plasticizing process of plastic granules and also reduce the quality of the processed material due to enclosed water vapor. Therefore, most thermoplastic granules are dried by drying air and with varying energy expenditures. The drying capacity depends on the initial and the final moisture and on the through put rate of the drying chute. In order to avoid problems during the drying process due to varying properties of the thermoplastic materials, different drying conditions are required so as not to damage the thermoplastic material. Especially the new so-called "high-tech" plastics now introduced to the market are very sensitive and the borderline between optimally drying and damaging or altering the material is very narrow. The damages or alterations, for example, the excretion of additives or the destruction of the polymer network, result from extended drying periods and/or drying temperatures chosen too high. The quality and the required time of the drying process are a function of the dew point, the air flow, the drying time and the drying temperature.
It has been known to adjust the drying process for plastic granules by adjusting the air flow. Plastic granules that have been heated once maintain their drying temperature even when the air flow is reduced as long as the heat loss of the material is not greater than the energy introduced by the reduced air flow. A natural cool-down of the granules is not desired and the drying chute should be well insulated in order to conserve energy. Since the drying temperature is maintained at a constant level even though the air flow is reduced, damage to the plastic granules is almost inevitable.
It is therefore an object of the present invention to provide a method and an apparatus of the aforementioned kind that ascertains a reliable and energy conserving drying of the material without damaging the material.